Problems in Quantification of Pitting Corrosion
Jürgen Gelz1, Muhammad Yasir2, Gregor Mori2, Burkhard Meyer3, Helmut Wieser4
1
Hochschule Aalen, Germany
2
Christian Doppler Laboratory of Localized Corrosion, University of Leoben, Austria
3
P-A-M Röntgenfilmdigitalisierung, Germany
4
EMCON Technologies GmbH, Germany
Abstract
Pitting corrosion is a localized type of corrosion in the form of holes that can penetrate in
materials extremely rapidly. It is one of the most frequent forms of corrosion in stainless
steels, mainly caused by chlorides. Quantification of pitting attack has been the focus of
intense inquiry in recent years. Pitting corrosion is quantified in different ways e.g. average
pit depth measurement, maximum pit depth measurement, remaining wall thickness and
affected area due to pitting.
Measurement of average pit depth is complicated by the fact that there is a statistical variation
in depth of pits. There is always a large variation in pit size on the same specimen so
interpreting results in terms of average pit depth is critical. Measurement of the maximum pit
depth is also not a reliable way to express the pitting damage. In this paper, advantages and
disadvantages of some used methods of quantitative characterization of pitting corrosion are
explicated. A detailed comparison of three methods, i.e. optical microscopy, 3D optical
microscopy and X-ray radiography have revealed that high resolution X-ray radiography is a
powerful and easy method to quantify pitting corrosion both in terms of pit depths and
percentage of affected area. This method overcomes common problems like high scatter of
results, high personal and cost efforts with low reproducibility for other methods. Principle,
accuracy, merits and limitations of the proposed methodology are discussed.
Keywords: Pitting corrosion, pit depth, optical microscopy, 3D optical microscopy, x-rayradiography.
Introduction
Pitting corrosion is one of the most prevalent forms of localised corrosion that is a dangerous
phenomenon because it is difficult to detect and predict. The attack is highly localised and pits
can penetrate inwards extremely rapidly and the deepest of them can damage the structure by
perforating the material. The pitting corrosion of stainless steels is the most important
corrosion processes that affect the service behaviour of these materials [1]. Weight loss and
wall thickness reduction is not an appropriate and trustworthy way to interpret the pitting
corrosion because pitting corrosion is a localised form of attack. Quantification of pitting
corrosion is very important for understanding the behaviour of material and help to measure
the corrosion distribution leading to a better understanding of how the material is affected
during its service. This also helps to develop a ranking for a choice of the right material for a
specific application.
There are various ways to quantify the pitting corrosion attack e.g. average pit depth
measurement, maximum pit depth measurement, pitting density (pits/cm2) and remaining wall
thickness due to pitting. A detailed comparison of three techniques, i.e. classical optical
microscopy, 3D optical microscopy and X-ray radiography was done during this research.
The application of a recently modified radiographic technique for corrosion quantification is
described in this paper. This system has some possible advantages compared to other
conventional quantification methods.
X-ray radiography:
Digital radiography is one of the oldest NDT techniques, which is in use since long time ago
as a monitoring method to detect major flaws and severe corrosion attacks in many industrial
systems. X-rays are generated by means of specially designed high vacuum tubes (X-ray
tubes). When electrically operated this tube emits penetrating radiation beams known as Xrays. The beam from equipment penetrates a piece of metal, and the amount the beam is
attenuated depends on the thickness of the material, and hence the intensity of the transmitted
beam varies with position. Step wedges are used for the calibration and standardization of Xray machines. Also, when an object with various thicknesses is radiographed, a step wedge of
the same material is used. The resulting radiograph shows the dimensional features of the part.
In a photograph, the thinnest portion of a sample will cause dark image while the thickest
portion where the intensity was low will cause a bright image. The principle of digital
radiography is shown in figure 1.
X-ray tube
X-ray radiations
Radiography
film
Figure 1: Radiographic image of calibration sample and corroded sample [3]
In this work specimens from cyclic corrosion tests were investigated with the help of this
method. Figure 2 shows a coupon and its radiographic images with two different filters.
Figure 2: Corrosion coupon with its radiographic images
The figure 3 shows an example of quantification of corrosion attack in terms of percentage
affected area and reduction in wall thickness of a specimen due to corrosion attack.
108
10
1
0
> x mm [%]
corroded area with a wall thickness reduction of
100
0,1
0,2
0,3
0,4
0,5
0,6
0,1
108
0,01
0,001
0,0001
0,00001
w all thickness reduction[m m]
Figure 3: Wall thickness reduction VS percentage infected area
The probability of the pits that initiate on a specimen surface is calculated by using statistical
extreme value distribution method. The development of extreme value theory for engineering
applications has provided the mathematics to predict the deepest corrosive attack in a large
structure from a limited number of measurements on a portion of the structure. An example of
extreme value distribution plot of maximum pits is shown in figure 4.
108
extreme value function of
probability
4
3
2
108
1
0
-1
-2
0
0,02
0,04
0,06
0,08
0,1
0,12
0,14
pit de pth [m m ]
Figure 4: Extreme value distribution plot of the deepest pit data
3D-optical microscope:
The use of 3D optical microscope to quantify pitting corrosion is also common now a day.
Using these measuring tools enables quantification of corrosion damage with high resolution
visualisation. In this research an Infinite Focus Measurement Machine (IFM) was used to
measure pit depths and to obtain 3D data set from a sample surface. Like other pitting
quantification methods it’s also a non-destructive method and in use in many corrosion
laboratories around the world. Its operating principle is Focus Variation. This moves the
focus plane of a known objective lens vertically over a surface which in conjunction with
Smartflash illumination builds up a 3D full colour model of the surface to be examined. This
provides the 3D topography of the analyzed surface, which enables to distinguish between
corroded and un-corroded areas of the specimen. The IFM method allows capturing images
with a lateral resolution down to 400 nm and a vertical resolution down to 10 nm. In this
research entire surface of specimens were evaluated by an automated 3D optical microscope
with statistical data analysis. Characterized parameters were the maximal pit depth and pit
volume. An example of pit depth measurement with this method is shown in figure 5.
Figure 5: Measurement of pit depth with 3D microscope [3]
Conventional Optical Microscopy:
The use of conventional optical microscope to measure pit depths is one of the well known
oldest method and still not exhausted [4]. During this study the pit depth distribution was
measured by using an optical microscopy with a calibrated eyepiece. The sample was marked
into equal parts and pit depths were measured in each section. In a first step a single pit was
located on the specimen’s surface. The pit depth was a difference in height of a sample
surface and bottom of a pit measured by focusing microscope’s knob as shown in figure 6.
Depths of pits were measured one by one with this technique and normally results were
interpreted in terms of the maximum pit depth or the average of the ten deepest pits. High
personal efforts are required to locate and measure depths of pittings on a whole surface of a
specimen. The rate of accuracy in measurements is also not very high due to less resolution
and personal errors. Optical microscope (Olympus BX51M) was used in this study to measure
pit depths on steel coupons after corrosion tests.
Surface
200 X Magnification
∆=h
Figure 6: Schematic approach of measuring pit depth with optical microscope
Conclusion:
The following conclusions can be drawn from this study that each of the explained methods
has its own characteristics and advantages depending on the type and purpose of required
information. The comparison has revealed the fact that none of the discussed method fulfils
all the desired requirements. Every technique has its own merits and demerits but X-ray
radiography appears amongst one of the suitable especially in terms of personal efforts and
costs. The summary of all three methods in a form of comparison are explained in a table 1.
Size of
specimen
(700X700 mm)
Light Microscopy
3D Microscopy
X-ray Radiography
Costs
Low
High
Medium
Time required
Medium (1hr, dependent
on corroded surface)
Medium (1hr, dependent
on corroded surface)
Less (30 mins, independent
on corroded surface)
Possible
Analysis
Pit depth measurement
2D photographs
Percentage affected area
2D & 3D photographs
Percentage affected area
2D photographs
Precision &
Accuracy
Pit depths: Medium
(Till 20 µm exactness)
Pit depths: High
(Till 1µm exactness)
Pit depths: High
(Till 5 µm exactness)
Reproducibility
Bad
(Examiner dependent)
Very Good
Good
Table 1: Comparison of light microscopy, 3D microscopy & radiography
Low cost and time are obvious advantages of this technique however it has some limitations
also. Sometimes, precise estimation of a pit is also difficult because of alignment of the pit
relative to radiation beam. The most significant limitation of this technique is accessibility of
both sides of the sample to X-ray radiations. The results of this work have shown that
radiography is a very promising way for quantification of pitting corrosion both in terms of
maximum pit depth and percentage of infected area due to corrosion.
The results obtained by this method are fully satisfactory and on the basis of them
susceptibility to pitting corrosion for each material can be ranked. The quantification of
corrosion allows comparing and ranking different grades which lead to improve the selection
of the right material.
References
1. Mars G. Fontana: Corrosion Engineering, Third Edition, Tata McGraw-Hill.
2. Harmuth Schroettner, Mario Schmied and Stefan Scherer, “Comparison of 3D surface
reconstruction data from certified depth standards obtained by SEM and an infinite focus
measurement machine (IFM)” in April 2006.
3. Jürgen Gelz, “Qualification of a new stainless steel for automotive exhaust applications”
Diploma thesis, Hochschule Aalen, submitted in August 2008.
4. Standard guide for examination and evaluation of pitting corrosion, G 46-94, ASTM
Standards 2006.
Acknowledgements
This study is part of a research program financed by the Christian Doppler Organization and
EMCON Technologies and the authors would like to thank Mr. Martin Stüttem for his kind
support and permission to publish this work.